Optical modulation by submillimeter-wave signals and applications thereof

ABSTRACT

A continuous-wave cyanide (HCN) laser has been employed to drive a continuous, traveling-wave lithium niobate (LiNbO3) electrooptic modulator at signal frequencies of 964 gHz. (311 Mu m.) and at 891 gHz. (337 Mu m.). Part of the power of a visible 0.633- Mu m. carrier beam is converted into two visible sidebands, one of which is favored by the choice of phasematching angle. In a submillimeter communication system and in power metering of submillimeter radiation, angular separation of the beams makes attractive the up-conversion to the visible region, followed by detection. The large frequency separation of the visible carrier and the sidebands facilitates precision spectroscopic and metrological applications.

Unite States Patent Bridges et al.

[451 Feb. 22, 1972 [54] OPTICAL MODULATION BY SUBMILLIMETER-WAVE SIGNALSAND APPLICATIONS THEREOF OTHER PUBLICATIONS Boyd, Up- Conversion of10.6u Radiation to the Visible" Pg. 515-519, QE. QE-4,No. 9, 9/68.

Frenkel, Methods for the Measurement of y and Stability of...in the LR.Pg. 883, E & E abstracts, No. 15359 9/69.

Primary Examiner-Rodney D. Bennett, Jr. Assistant Examiner-N. MoskowitzAttorneyR. J. Guenther and Arthur J. Torsiglieri [57] ABSTRACT Acontinuous-wave cyanide (I-ICN) laser has been employed to drive acontinuous, traveling-wave lithium niobate (LiN- bO electro-opticmodulator at signal frequencies of 964 gl-Iz. (311 um.) and at 891gl-iz. (337 pm). Part of the power ofa visible 0.633-um. carrier beam isconverted into two visible sidebands, one of which is favored by thechoice of phasematching angle. In a submillimeter communication systemand in power metering of submillimeter radiation, angular separation ofthe beams makes attractive the up-conversion to the visible region,followed by detection. The large frequency separation of the visiblecarrier and the sidebands facilitates precision spectroscopic andmetroiogical applications.

6 Claims, 5 Drawing Figures RECEIVER [72] Inventors: Thomas JamesBridges, Holmdel; Ivan Paul Kaminow, New Shrewsbury; Martin AlanPollack, Colts Neck, all of NJ.

[73] Assignee: Bell Telephone Laboratories, Incorporated,

Murray Hill, NJ.

[22] Filed: May 4, 1970 [21] App1.No.: 34,198

[52] US. Cl ..332/7.51,250/199 [51] Int. Cl ..I-I01s3/l0 [58] Field ofSearch ..330/4.3; 332/751; 250/199; 350/160 P [56] References CitedUNITED STATES PATENTS 3,555,455 1/1971 Paine ..332/7.5l 3,492,492 1/1970Ballman.. .....250/199 3,413,476 11/1968 Gordon ..332/7.51

TRANSMITTER A SUBMILLIMETER 75 WAVE MODULATOR LASER 5 TRANSMISSIONMEDIUM 77 SIGNAL 76 SOURCE 5 B7 SIGNAL our B1 REFRINGENT ELECTROOPTICCRYS2TAL OPTIC AXIS ORTHOGONAL TO PLANE OF PAPER BEAM 35 SCANNING 36 3337 34 DRIVE LASER FOR MIRROR u UPPER SlDEBAND Y Q BEAM 20 2 FIG. 19 EBACK A REFLECTOR FRONT 23 REFLE4CTOR I I .F, ,BIREFRINGENT 30/-ELECTROPTIC 3: I CRYSTAL ll '2 1 OPTIC AXIS ORTHOGONAL TO L L 41fPLANE OF PAPER y r I I l I 25 26 SUBMILLIMETER -WAVE LASER I3 R-F 3sPUMPING 34 SOURCE 35 VISIBLE-WAVE LASER I! gwiww ATTORNEY PATENTEDFEBZEI972 3,644,848

sum 2 or 4 FIG. 2

5 5? Wm mm ksz 53 w w w LATTICE FREQUENCY RESONANCES BACKGROUND OF THEINVENTION This invention relates to optical modulators, particularlythose in which the modulating signal is in the submillimeter range.

An optical modulator is a modulator in which a beam of near infrared,visible or ultraviolet electromagnetic radiation is caused to vary insome detectable way in response to a modulating signal, typically aninformation-bearing signal. The submillimeter range is the range from300 gHz. to 3,000 gI-lz. (1,000 zm. to 100 zm. wavelength).

While wideband modulation of optical beams by microwave signals guidedby a waveguide has heretofore been proposed in an article by one of us(Mr. Kaminow), with Mr. W. W.

Rigrod, in the Proceedings ofthe IEEE, l 1 (Jan., 1963), this techniquehas not been successfully applied when the modulating signal is in thesubmillimeter range.

Moreover, since submillimeter wave sources are relatively few and weakand detectors are relatively slow and insensitive, this region of thespectrum has been relatively forbidding with respect to practicalapplication. Nevertheless, if these problems could be overcome,submillimeter waves could become attractive for communication systems ofgreater capacity than microwave-waveguide systems.

SUMMARY OF THE INVENTION Our invention is based upon the discovery ofeffective optical modulation driven by a freely propagating coherentsubmillimeter-wave signal beam in a birefringent electro-optic crystal.No signal electrodes and no guiding structure are required.

More specifically, our discovery includes the first modulation of avisible coherent light beam by a freely-propagating beam from asubmillimeter-wave laser. In out experiments, the submillimeter-wavelaser is a hydrogen cyanide (I-ICN) laser employed to drive atraveling-wave lithium niobate (LiNbO electro-optic modulatorcontinuously at signal frequencies of 964 gHz. (311 pm.) and 891 gI-iz.(337 pm.) to convert part of the power of a 0.633-um. visible laser beaminto two visible sidebands, one of which is favored by the choice ofphasematching angle.

According to our invention, a coherent carrier beam in the optical rangeis modulated in a birefringent electro-optic crystal by a freelypropagating submillimeter-wave beam at a phase-matching anglesufficiently large to make the optical carrier and both opticalsidebands optically resolvable on account of their angular divergence.

According to first and second features of our invention, the angulardivergence of optical carrier and sidebands is employed, respectively,in up-conversion followed by visible-0ptical detection of a modulationfirst imposed upon the submillimeter-wave beam and in power metering ofsubmillimeterwave radiation.

Advantageously, the large frequency separation of the visible carrierand the sidebands facilitates precision spectroscopic and metrologicalapplications. Metrology is the science of measurement.

BRIEF DESCRIPTION OF THE DRAWING Further features and advantages of ourinvention will become apparent from the following detailed description,taken together with the drawing, in which:

FIG. 1 is a partially pictorial and partially block diagrammaticillustration of a modulator embodiment of the inventron;

FIGS. 2 and 3 show relationships pertinent to the operation of all ofthe embodiments of the invention;

FIG. 4 is a partially pictorial and partially block diagrammaticillustration of a power-metering embodiment of the invention; and

FIG. 5 is a partially pictorial and partially block diagrammaticillustration of a communication system employing the invention in areceiver.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS In the embodiment of FIG. 1, itis desired to modulate a 0.633-pm. visible laser beam from thehelium-neon laser 11 with a submillimeter-wave laser beam from thesubmillimeterwave cyanide laser 13 to obtain a visible sideband beamthat is selected and passed to the output by a filter 21. The resultingbeam is useful in that it is shifted in frequency from that of the0.633-p.m. beam by the frequency of the submillimeter-wave beam and alsohas any amplitude modulation or other modulation present on thesubmillimeter-wave beam. A photomultiplier can then provide rapid,sensitive detection of this sideband whereas no comparabledirect-detection equipment for submillimeter waves presently exists.

The modulation is accomplished in the birefringent electrooptic crystal12, which is illustratively a lithium niobate (LiN- bO single crystalhaving its optic axis normal to the plane of intersection of the0.633-pm. beam and the submillimeterwave beam.

The 0.633-um. beam from laser 11 is directed through crystal surface 16,illustratively substantially normal thereto, by reflector 14 and lens 15disposed between laser 11 and crystal 12.

The submillimeter-wave beam from laser 13 is directed through crystalsurface 41, illustratively substantially normal thereto, by lens 18.

The angle 7 between crystal faces 16 and 41 is chosen to satisfy thesimultaneous conditions (1) that the 0.633-um. visible carrier beam isinternally reflected at surface 41, and also at the parallel surface 42,and (2) that the 0.633-;im. beam and the submillimeter-wave beamthroughout their region of intersection, are substantially phase-matchedto, for example, the upper sideband visible beam.

When both input beams are orthogonal to their entry surfaces, it can beshown from the law of reflection and principles of geometry that theangle between the directions of propagation of the input beams in theirregion of intersection is also approximately 7. Since in this instancethey are phasematched to the upper sideband, this angle will be called 7If the lower sideband beam were phase-matched to the input beams, thisangle would be termed 'y.

The 0.633-pm. carrier beam and the phase-matched sideband beam areextracted through surface 17, which is parallel to surface 16, and aredirected by reflector 19 and lens 20 through a narrow-passband filter 21for selecting the upper sideband for further transmission to theeventual utilization apparatus (not shown).

In order to phase-match the lower sideband to the input beams, reflector14 is driven by piezoelectric element 39 mounted thereon, and anelectrical scanning drive source 40, to change the angle of propagationof the 0.633-p.m. beam through lens 15 and surface 16. A puretranslation of reflector 14 displaces the beam from the center of lens15 and changes its angle of propagation with respect to the axis of thelens. Alternatively, a mechanical scanning drive source 40 could movereflector 14 directly.

The 0.633-p.m. helium-neon laser 11 is of conventional type supplying aIS-milliwatt beam focused to a waist of about 4X 10- centimetersdiameter in the region of intersection h tbsh ami romla e 31, ,7 H

The lithium niobate crystal 12 has a thickness 1 between surfaces 41 and42, where t"=a and a is the absorption coefficient of the crystal in thesubmillimeter range for fields polarized along the optic axis. Thelength of crystal 12 along either surface 41 or 42 is w, where willustratively is 3.68 millimeters.

The submillimeter-wave hydrogen cyanide laser 13 comprises an activemedium of hydrogen cyanide gas in a suitable tube 28, 3 meters long,including the gold-coated back reflector 23, the polyethylene end window29 and the mesh-front reflector 24, which is about 98 percent reflectiveand about 2 percent transmissive, for the submillimeter-wave stimulatedradiation. The laser pumping apparatus includes the cathode 30, theanode 31 and the direct current voltage source 32 connected in seriesfor establishing a discharge between electrodes 30 and 31 to excite thehydrogen cyanide gas.

The front reflector 24 illustratively includes the two meshes 25 and 26fabricated and spaced to form a highly reflective (about 98 percent)submillimeter interference filter. The mesh wires are l9- m. wide copperwires on 102-,um. centers in a square array. The spacing of the meshes25 and 26 illustratively is approximately 167 p.m., and is finelyadjusted during fabrication or during laser operation to obtainreflectivity and transmissivity in the desired proportions.

The diameter of tube 28 is approximately 3 inches; and 20- centimeterlens 18 focuses the submillimeter-wave beam to a waist of approximately1.9-mm. diameter between its halfpower points in the vicinity of surface41 of crystal l2.

Illustratively, the filter 21 can include an input member 22 having aslit or aperture through which the selected beam passes. Such an inputmember is advantageous because of the angular divergence among all threeof the carrier beam and the sideband beams.

In the operation of the embodiment of FIG. 1, submillimeter-wave laser13 illustratively supplies about /2 watt at either 964 gI-Iz. (3 1 l m.)or 891 gI-Iz. (337 ,um.).

A l-db. attenuator (not shown) may be introduced at the output of laser13 to isolate it from crystal and lens reflections.

The upper sideband is illustratively phase matched by driving reflector14 until the upper sideband is detectable at the output of filter 21.This upper sideband beam propagates at a somewhat greater angle withrespect to the normal to surface 41 than the angle 'y+, the angle ofpropagation of the carrier beam.

Alternatively, if the lower sideband beam is phase matched by drivingreflector 14 until that sideband is passed by filter 21, that sidebandwill propagate at a smaller angle with respect to the normal of surface41 than the angle -y (not shown), which is the angle of propagation ofthe carrier beam under those conditions. It will be recalled that thecarrier beam will not be normal to surface 16 during at least one ofthese regimes and that the position of reflector l4 and the focusingproperties oflens determine this angle.

The modulation characteristics of the embodiment of FIG. 1 can bedescribed as follows.

Utilizing the r electro-optic coefficient in LiNbO i.e., both thevisible carrier and submillimeter-wave modulating waves polarized alongthe c-axis, the phase modulation index 18 11 1 1) xi 2 im for small 1;,with J,,(1;) the nth order Bessel function. The sideband spectrum isillustrated in FIG. 2.

Phase velocities of the interacting waves must be matched over the path[specified in Equation (1) for each sideband frequencyw separately,

where k is the wavevector. If the angle between k and k,,, is y for eachsideband, as in FIG. 3, then for (u /w small, with 6 the dielectricconstant along the c-axis at w,,,, and y y Taking e =27 and n 2.2Oyields y=65 and (y 'y )=0.51. If the matching condition is not exactlysatisfied, then Equation (1) is reduced by a (sin factor that has itsfirst Zero when the angular deviation is in the radial direction, with w=1.6 mm. The field decays as with 01 1] emf. Because of surfacereflections, the modulating power P entering the crystal is TP, withT=0,54 and P the incident power. The field at z=O, r=0 is where z =377ohm. According to Equation (6), E,,,(O)=440 P v/m with P, in watts.Since 01 F095, multiple reflections can be neglected. Note that adjacentsegments of the optical beam path are not modulated because the matchingcon dition is not satisfied. After some manipulation the integral inEquation (1) becomes 9.6Xl0 E,,,(0) volts. For comparison, the opticalpath length I is 20 10"m. Then, with the radiofrequency electro-opticcoefficient r =31 1O m/v, the calculated 17 and R are n 7.0 x 10 m (rad)Repeated sideband ratio measurements with P, '=0.l W give at either 964or 891 gHz. The good agreement confirms the invariance of r up to 964gI-lz. The chief experimental uncertainties arise from measurement of P,50 percent) with an Eppley thermopile having a reflection coefficient ofpercent at 337 um, and from measurement of optical attenuation at 0.633,um. 20 percent). The angles predicted in Equations (4) and (5) werefound to be 'y=63, -y y,=O.4 and 5y= 1. These are internal angles thathave been corrected for refraction at the surface. The observed 'y,which is significantly smaller than the calculated value, implies e 24if n 2.2.

Operating characteristics of the device at other carrier and modulationfrequencies can be determined by considering the wavelength dependencein Equation (1) as well as dispersion in a 6 and n The foregoingmeasurements of sideband ratio suggest a technique for power-meteringsubmillimeter-wave radiation more sensitively and more rapidly thanheretofore possible. A modified embodiment for this purpose isillustrated in FIG. 4.

In FIG. 4, components numbered the same as in FIG. 1 are substantiallythe same as those components of FIG. 1.

The principal change in the embodiment of FIG. 4, as compared to theembodiment of FIG. 1, is that filter 21 is replaced by the assembly 60designated a power meter. Power meter 60 includes the filter 71 whichpasses only the sideband frequency and rejects any carrier light thatmight have been scattered into that path. It further includes thecalibrated variable attenuator 72, illustratively a rotatable polarizerdisposed between fixed polarizers, and the slit device 62 having slitsdisposed for resolution of the angularly separated carrier and sidebandbeams. The photomultipliers 67 and 68 intercept the light passingthrough the respective slits and apply their output signals to ratiocircuit 70, which is a known type of electronic circuit for indicatingthe ratio of two slowly varying electrical signals and canillustratively be a so-called bridge circuit.

The attenuator 72 is varied to reduce the carrier power until the ratioindicated by circuit 70 is unity. The submillimeterwave power is thenread directly from calibrated attenuator 72. This measurement can beextremely precise because of the passive nature of attenuator 72.

The slit device 62 can illustratively include an absorber 63 in aposition to intercept the other sideband beam, which may be very weaklypresent despite the lack of phase-matching conditions for it. Absorber63 reduces scattering of this sideband into the path of the other beamsin power meter 60 and correspondingly alleviates the filteringrequirements for those beams.

A further modification of the embodiment of FIG. I to facilitate its usefor up-conversion and detection in the receiver of a communicationsystem is illustrated in FIG. 5. Here again, components numbered thesame as in FIG. 1 are substantially the same as described there.

Between front reflector 24 of laser 13 and lens 18 is disposed amodulator 75, which is illustratively a chopper-type modulator or anelectromechanical modulator capable of modulating the submillimeter-wavebeam at a moderate information rate. As a matter ofpractical interest,many crystalline phenomena could usefully be studied with the aid ofsubmillimeter-wave radiation and would provide such modulation ofscientific interest, particularly in spectroscopy. It is desirable tohave sensitive, fast means for detecting this modulation.

Such a detecting means is provided at a receiver separated from lens 18by a suitable transmission medium 77. The receiver includes laser 11,crystal l2 and associated apparatus, particularly that following lens20. That modified apparatus includes slit device 82, which passes thelower sideband beam at aperture 84 while rejecting the carrier,illustratively by absorption at absorber 83.

Device 82 is followed by the photomultiplier 87 and output amplifier 88.

In operation, the crystal l2 up-converts the received submillimeter-waveradiation to the visible portion of the spectrum, in which themodulation originally imposed on the submillimeter-wave beam is readilydetected by photomultiplier 87.

Especially for the case of an extended transmission medium 77, whichcould be a guiding structure for submillimeter radiation, it is readilyseen that our invention solves one of the problems of futuresubmillimeter communication systems.

Further applications of our invention include refinements in themeasurement of the velocity of light because of the submillimetermodulating frequencies that can now be used as a result of ourinvention. The basic apparatus in which our modulator could be employedfor this purpose is disclosed in the article, Locking a Laser Frequencyto the Time Standard, by Z. Bay and G. G. Luther, Applied PhysicsLetters, Vol. 13, page 303 (Nov. 1, 1968). In that article, the problemthat higher modulating frequencies are needed is pointed out, withoutsolution.

In that apparatus as well as precision spectroscopic applications andmetrological applications, our modulator is useful because of the largefrequency separation between carrier and sidebands. The frequencyseparation can be measured very accurately by mixing a portion of thesubmillimeter-wave radiation with harmonics of radiofrequency standardsby any of several known techniques. One such technique is that disclosedby L. O. I-Iocker et al., Applied Physics Letters, Vol. l0, page 147(1967).

The following modification of our invention is also within its scope.If'y-lor -y is less than the angle required for total internalreflection in crystal 12, then the visible carrier beam may be incidenton the same face as the modulating beam. The two beams would be incidentat angles such that they would be refracted to propagate internally atthe relative angle 'y+ or y-.

We claim:

1. Optical modulation apparatus of the type comprising a source of afirst beam of optical radiation, a birefringent electro-optic crystalpassing said first beam in a first direction, said crystal havinglattice resonances in the far infrared range between 10 um. and um, andmeans for modulating said first beam at a wavelength longer than 100,um., said apparatus being characterized in that said modulating meansincludes a source of a second coherent beam in the submillimeter rangebetween 100 p.m. and 1,000 um. and means for directing saidsubmillimeter coherent beam into said crystal as a freely propagatingbeam in a second direction making an angle with said first directionproviding phase-matched interaction of said beams yielding at least onesideband beam substantially angularly separated and optically resolvablefrom said first beam.

2. Optical modulation apparatus according to claim I in which thebirefringent electro-optic crystal is shaped and adapted to pass thefirst beam in a first direction yielding multiple internal reflectionsof said first beam, said reflections lengthening said phase-matchedinteraction, and the modulating means includes means for changing theangle of internal reflection of said first beam within said crystal andmeans for filtering the optical radiation emerging from said crystal toseparate the phase-matched sideband beam from said first beam.

3. Optical modulation apparatus according to claim 2 in which the meansfor changing the angle of internal reflection is capable of selectingeither the upper sideband beam or lower sideband beam for phase-matchedinteraction with the first and second beams.

4. Optical modulation apparatus of the type comprising a source of abeam of visible optical radiation, a birefringent electro-optic crystalpassing said visible beam in a first direction yielding multipleinternal reflections and no double refraction of said visible beam, saidcrystal having lattice resonances in the far infrared range between 10pm. and 100 um, and means for modulating said visible beam at awavelength longer than 100 pm said apparatus being characterized in thatsaid modulating means includes a source of a coherent beam in thesubmillimeter range between 100 um. and 1,000 in. and means fordirecting said submillimeter coherent beam into said crystal as a freelypropagating beam in a second direction making an angle with said firstdirection providing phase-matched interaction of said beams yielding twosideband optical beams substantially angularly separated and resolvablefrom said visible optical beam.

5. Optical modulation apparatus according to claim 1 in which themodulating means includes variable calibrated means for attenuating theportion of the first beam emerging from the crystal and means forderiving the ratio of said one sideband beam to said attenuated portionof the first beam, whereby variation of the attenuating means to yield afixed ratio yields an indication of submillimeter-wave power level.

6. Optical modulation apparatus according to claim 1 in which the sourceof the second beam includes means for imparting a modulation to saidsecond beam, the means for directing said second beam includes atransmission medium,

1. Optical modulation apparatus of the type comprising a source of afirst beam of optical radiation, a birefringent electrooptic crystalpassing said first beam in a first direction, said crystal havinglattice resonances in the far infrared range between 10 Mu m. and 100 Mum., and means for modulating said first beam at a wavelength longer than100 Mu m., said apparatus being characterized in that said modulatingmeans includes a source of a second coherent beam in the submillimeterrange between 100 Mu m. and 1,000 Mu m. and means for directing saidsubmillimeter coherent beam into said crystal as a freely propagatingbeam in a second direction making an angle with said first directionproviding phase-matched interaction of said beams yielding at least onesideband beam substantially angularly separated and optically resolvablefrom said first beam.
 2. Optical modulation apparatus according to claim1 in which the birefringent electro-optic crystal is shaped and adaptedto pass the first beam in a first direction yielding multiple internalreflections of said first beam, said reflections lengthening saidphase-matched interaction, and the modulating means includes means forchanging the angle of internal reflection of said first beam within saidcrystal and means for filtering the optical radiation emerging from saidcrystal to separate the phase-matched sideband beam from said firstbeam.
 3. Optical modulation apparatus according to claim 2 in which themeans for changing the angle of internal reflection is capable ofselecting either the upper sideband beam or lower sideband beam forphase-matched interaction with the first and second beams.
 4. Opticalmodulation apparatus of the type comprising a source of a beam ofvisible optical radiation, a birefringent electro-optic crystal passingsaid visible beam in a first direction yielding multiple internalreflections and no double refraction of Said visible beam, said crystalhaving lattice resonances in the far infrared range between 10 Mu m. and100 Mu m., and means for modulating said visible beam at a wavelengthlonger than 100 Mu m., said apparatus being characterized in that saidmodulating means includes a source of a coherent beam in thesubmillimeter range between 100 Mu m. and 1,000 Mu m. and means fordirecting said submillimeter coherent beam into said crystal as a freelypropagating beam in a second direction making an angle with said firstdirection providing phase-matched interaction of said beams yielding twosideband optical beams substantially angularly separated and resolvablefrom said visible optical beam.
 5. Optical modulation apparatusaccording to claim 1 in which the modulating means includes variablecalibrated means for attenuating the portion of the first beam emergingfrom the crystal and means for deriving the ratio of said one sidebandbeam to said attenuated portion of the first beam, whereby variation ofthe attenuating means to yield a fixed ratio yields an indication ofsubmillimeter-wave power level.
 6. Optical modulation apparatusaccording to claim 1 in which the source of the second beam includesmeans for imparting a modulation to said second beam, the means fordirecting said second beam includes a transmission medium, the source ofthe first beam is a source of a visible beam of optical radiation, andthe means for modulating the first beam is adapted as an upconvertingdetector for the modulation imparted to the second beam, in that saidmeans for modulating said first beam includes means for spatiallyfiltering the one sideband beam from said first beam and photocathodemeans for detecting modulation upon said sideband beam.